Large plate filters employing paper filter elements are often used for filtering operations in which the object is to cleanse large quantities of liquid, for example cleaning oil used during metal rolling operations or during metal can manufacture. The filtering units are relatively expensive and so is the non-reusable filter paper employed by them. Changing filter paper can also be quite time-consuming.
One way to reduce the cost and time associated with changing filter paper is to use filters that can be backwashed to remove the contaminants deposited thereon. A problem associated with backwashing is that if a pump is used to do so, the head which the pump must develop for backwashing may be many times greater than is necessary for the filtering operation. It has been found that the use of a volume of compressed air to force a portion of the liquid in the filtering apparatus back through the filter element works well to dislodge the filter cake. This method eliminates the need for a larger pump solely for backwashing. U.S. Pat. No. 2,423,172 to Booth discloses backwashing by means of compressed air. However, Booth merely discloses to provide compressed air which is used to force backwash water in a counterflow direction through the filter. No consideration is given to the volume of air that is employed, or to changes in the volume of air due to changes in the pump pressure or a change in pressure drop across the filter element. Depending on the pump pressure and the pressure drop across the filter element when the filter is backwashed, the amount of backwashing liquid that is forced through the filter is essentially uncontrolled. To prevent backwashing liquid from being ejected from the filter it was necessary to precisely time the operation of a dual valve system. Failure to properly sequence the valves could lead to a discharge of liquid from the filter which is highly undesirable and could lead to injuries to persons and damage to surrounding property unless adequate safeguards are provided.
Other patents which may be of interest are U.S. Pat. Nos. 3,056,499; and 3,280,978.